Geophysics is the study of the physics of the Earth, especially its electrical, gravitational, and magnetic fields, and the propagation of the elastic (i.e. seismic) waves within it. A major part of the search for oil and gas requires information concerning the geological features where hydrocarbons may be trapped.
Seismic surveying allows for mapping of subsurface distribution of different types of rocks and the fluids they contain. Oil field seismology, which accounts for approximately 95 percent of all geophysical activities worldwide, has traditionally been applied from the surface of the earth. The source and receivers of a surface seismic survey are usually positioned on the surface (or close to it), because surface equipment is the easiest to deploy.
Originally, land based seismic surveys were conducted using explosives as an energy source. These had advantages because the explosives were lightweight, could reach remote locations and provided a strong penetrating signal having a compact wavelet with wide bandwidth. However, explosives are dangerous, cannot be used in populated areas, and the survey geometry is not easily update since shot holes have to be drilled for every placement. Thus, explosives are no longer as popular as they once were.
Vibroseis™ energy sources are the most common seismic source options for onshore hydrocarbon exploration. The generic term “vibrator” refers to these types of seismic sources, which were initially developed by Conoco in 1952 (see e.g., U.S. Pat. No. 2,688,124). The vibrator started life as an alternative to explosive sources on land. It had to compete for a short time with weight-drop trucks, but they could not be synchronized, whereas vibrators could (see e.g., FIG. 1). After various technologies were tested, hydraulic technology was found to be the most suitable for use in geophysics. In 50 years, the land vibrator weight, which is related to signal amplitude, increased from 10,000 to 90,000 lbs.
Vibrators have several features that make them attractive for seismic data acquisition. They are mobile and allow efficient and expeditious illumination of subsurface targets from many different shotpoint locations. Also, the frequency content of a vibrator signal often can be adjusted to better meet resolution requirements needed for a particular target. In addition, the magnitude of the energy input into the Earth can be tailored for optimal signal-to-noise conditions by varying the size and number of vibrators or by altering the output drive of individual vibrators. For these reasons, vibrators are one of the most versatile onshore seismic energy sources.
Vibrators work on the principle of introducing a user-specified band of frequencies, known as the “sweep,” into the Earth and then cross-correlating that sweep function with the recorded data to define reflection events. The parameters of a vibrator sweep are:                Start frequency        Stop frequency        Sweep rate        Sweep length        
A vibrator can do an upsweep that starts with a frequency as low as 1 to 2 Hz and stops at a high value of 100, 150, or 200 Hz. Alternatively, vibrators can do a downsweep that starts with a high frequency and finishes with a low frequency. Most Vibroseis data are generated with upsweeps, however, due to ghosting issues and operational issues in the hydraulics controller.
Sweep rate can be linear or nonlinear. A linear rate causes the vibrator to dwell for the same length of time at each frequency component. Nonlinear sweeps are used to emphasize higher frequencies because the vibrator dwells longer at higher frequencies than it does at lower frequencies.
Sweep length defines the amount of time required for the vibrator to transverse the frequency range between the start and stop frequencies. As sweep length is increased, more energy is put into the Earth because the vibrator dwells longer at each frequency component. Sweep length is usually in the range of 4 to 60 seconds.
If a vibrator sweep is 12 seconds long, then each reflection event also spans 12 seconds in the raw, uncorrelated data, plus some listen time to allow the signal to propagate into the earth, reflect and return. It is not possible to interpret uncorrelated Vibroseis data because all reflection events overlay each other and individual reflections cannot be recognized.
The data are thus traditionally reduced to an interpretable form by a cross-correlation of the known input sweep with the raw data recorded at the receiver stations. Each time the correlation process finds a replication of the input sweep, it produces a compact symmetrical correlation wavelet centered on the long reflection event. In this correlated form, vibroseis data exhibit a high signal-to-noise ratio, and reflection events are robust wavelets spanning only a few tens of milliseconds. More modern techniques for designaturing the data involve inversion, deblending and wavefield reconstruction methods.
As a general observation, if an area is plagued by random noise, vibrators are an excellent energy source because the correlation process used to reduce the vibrator sweep to an interpretable form discriminates against noise frequencies that are outside the source sweep range. If several sweeps are summed, unorganized noise within the sweep range is attenuated (the noise cancels itself out), while the true signals are amplified. However, if coherent noise with frequencies within the vibrator sweep frequency range is present, then the correlation process may accentuate these noise modes.
Probably the most important improvement in vibrator operations has been the development of ground-force phase-locking technology. Application of this technology results in the same ground-force function (i.e., the same basic seismic wavelet) being generated during hundreds of successive sweeps by vibrators positioned over a wide range of ground-surface and soil conditions and by all vibrators in a multivibrator array. All aspects of seismic data processing benefit when a source generates consistent output wavelets throughout a seismic survey, hence the appeal of vibrators as the source of choice for most onshore surveys today.
All seismic energy sources generate both surface waves and body waves. However, in seismic surveys, geophysicists are mainly interested in the body wave, since the desire is to visualize the interior structure of the reservoir. The two types of body waves are compressional (P) and shear (S) body waves (see FIGS. 2A and 2B respectively). P-waves are also called pressure or primary waves, and S-waves are also called secondary or transverse waves.
Shear waves have slower rates of propagation through the earth than do longitudinal waves, so they can produce a higher degree of resolution at a given frequency. This may enable the detection of subterranean anomalies that might otherwise be undetectable and the mapping of larger bodies with a higher degree of precision. Horizontally polarized shear waves are also less likely to be converted into different wave types upon interacting with horizontal interfaces, as is the case with compression waves, and accordingly seismograms made from such waves may be simpler to interpret.
Despite the recognized advantages of S-waves, the use of shear waves in seismic prospecting has been greatly limited because of the unavailability of suitable shear wave vibrator energy sources. This is due to the normally difficult problem of properly coupling the vibrator to the earth in shear mode. Without large teeth mounted to the shear vibrator base plate, the shear vibe tends to slide or “walk” off the source point and thus not input the correct signal into the ground. With large teeth, the vibe no longer walks, but may create unsightly holes or damage in the soil and cause difficulty with the surface landowner.
To study the physics and exploration applications of S-waves, it is often necessary to increase the amount of S-wave energy in the downgoing wavefield and to produce a shear wavefield that has a known vector polarization.
These objectives can be accomplished with sources that apply horizontally directed impulses to the Earth or by vibrators that oscillate their baseplates horizontally rather than vertically. In either case, a heavy metal pad is used to impart horizontal movement to the Earth by means of cleats on the bottom side of the pad that project into the Earth. A specific design for a horizontal shear-wave vibrator can be found in a patent issued to Fair (U.S. Pat. No. 3,159,232 Shear Wave Transducer).
Horizontal vibrators have also been improved with the introduction of ground-force phase-locking technology that results in more consistent shear wavelets from sweep to sweep as horizontal vibrators move across a prospect. Surface damage has been minimized by reducing the size of the cleats underneath the baseplate so that they make only shallow ground depressions.
Another method of generating shear waves is discussed in WO201312265, wherein tilted boreholes (15-45) are used together with directional detonations in an attempt to generate shear waves. If the borehole bottoms are near enough, the signals acquired in response to each firing can be combined (e.g., subtracted) to cancel the (non-directional) compressional wave information and reinforce the (directional) shear wave information. However, as noted above explosive use is sometimes not allowed. Further, the need for cased inclined boreholes limits the application of such technology.
Therefore, although several researchers have made efforts to design sources capable of providing shear waves, the results to date can still be improved. Shear wave energy sources have proven to be either infeasible or lacking in sufficient reproducibility, frequency band width, and power for repeated high resolution surveys for, e.g., reservoir monitoring. Thus, what is needed in the art are better methods of generating shear waves and preferably methods will also allow control over the direction and amplitude of shear waves, be highly reproducible from shot to shot, and of sufficient strength for deep penetration.